Here is the link:
I can't see the warehouse prices of the S21 series itself but usually 10% lower. Appears you can also get the $100/$150/$200 Samsung credit as well. No add a line necessary.
I think this deal is great for one of the older phones that qualifies for the $450 (such as Galaxy S8 or Iphone 8) and combine that with either the S21 or S21 Plus. If you're looking at the S21 Ultra and don't mind the EIP credits, than T-mobile deal is better.
Since this deal isn't live yet, please confirm all pricing with Costco on the S21 phones ( guessing $720 / $910 / $1080 ) and double check that the trade in rebate can for sure be combined with the Samsung Accessories credt BEFORE ordering.
QA Note: 1/22 - 1/28/21